Prior robotic devices for picking and placing components from one position to another included sub-assemblies for moving the component in a vertical or Z-axis direction. A prior robotic sub-assembly 10 is illustrated in FIG. 2. Such sub-assemblies included a case housing 12 having received therein an aluminum cylindrical barrel 14 capable of moving in a Z-axis direction (line Z in FIG. 1). The sub-assembly also included three sets of ball bearings 16 positioned at approximately 120.degree. from each other. The ball bearings are secured in position by modified dowel pins 18 extending through a longitudinal axis of the cylindrical ball bearing. The dowel pins have grooves for receiving retaining-rings 20 which hold the dowel pin in position. To position the double sealed ball bearings, it was necessary to mill out three small areas 22 having a width slightly larger than the longitudinal length of the cylindrical ball bearings. Further, in order to insert the modified dowel pins, three large areas 24 had to be milled out and bores drilled through the cylindrical ball bearing supports 26. The numerous processing steps for forming such prior sub-assemblies were undesirably expensive and labor intensive.
Further, the prior sub-assemblies had numerous disadvantages. The sub-assemblies lacked acceptable component location repeatability. The aluminum barrel experienced substantial wear and created substantial radial location error at the tool tip. The milled slots were also a source of rotational error which became worse with wear of the bearings. The bearings were directly exposed to the manufacturing environment which allowed them to become contaminated with dust and dirt producing "sticky" movement. Smooth movement was also hindered by bearing preload. When the barrel began to wobble due to wear, a load would have to be placed on the bearing by tightening a screw extending through the cylindrical bearing. Because the aluminum barrel wore unevenly, the bearing preload was adjusted resulting in a sacrifice of radial accuracy for smoother movement. The prior devices required frequent maintenance to adjust the bearing preload to accommodate for wear and to reduce radial error. These adjustments required total disassembly of the device which was complicated and time consuming.
Thus, heretofore there was a need for a robotic sub-assembly capable of producing zero error Z-axis compliance which was easy to manufacture, assemble, maintain and would have improved durability and wear resistance.